In the motor vehicle industry, it is conventional in an anti-lock hydraulic brake system to designate the master cylinder and the various equipment components which are either connected directly to or connectable with it as the "static circuit". Likewise, it is conventional to designate the booster valve equipment as well as the various equipment components which are either connected directly to or connectable with it as the "dynamic circuit".
Prior to the present invention, an anti-lock hydraulic brake system equipped with a power brake unit has been described in the prior art. See, for example, German publication No. DE 24 50 874 B2. This prior art reference teaches a master cylinder and the booster valve apparatus being combined into a single operating component. In this above-referenced publication, the servo cylinder provided is divided into a servo apparatus and a delivery cylinder. In this arrangement, therefore, the piston unit associated with the servo cylinder thereby comprises a piston of the servo apparatus and a piston of the delivery cylinder. The brake pressure required for operational movement of the wheel brake cylinder is produced in the brake chamber of the servo cylinder. This brake pressure consists of a first portion being produced by the static control pressure and a second portion being produced by the dynamic control pressure. The static control pressure is generated by means of a first control surface. This first control surface is the left surface of the piston associated with the delivery cylinder portion of the servo cylinder. The dynamic control pressure is generated by means of a second control surface. This second control surface is the left surface of the piston associated with the servo apparatus portion of the servo cylinder. In this prior art power brake, under normal operating conditions, the volume of the first control chamber of the servo cylinder which is delimited by such first control surface will increase with an increasing application force being exerted on the master cylinder. Such normal operating conditions, referred to above, exist when the anti-lock system (hereinafter referred to as ABS) is not engaged. The resulting stroke of the mechanism for making a brake application, which is normally a brake pedal, will transmit to an operator of the vehicle some feedback from the brake system, and thus the accustomed feel of the brake operation.
In the power brake disclosed in the above referenced German publication No. DE 24 50 874 B2, the ABS only acts on the dynamic control pressure by means of a corresponding electrically activated valve apparatus, which are designated 17 and 17a in such reference. Therefore, without additional measures being taken, only that portion of the brake pressure produced by this dynamic control pressure could be regulated during an operation of the ABS. Therefore, in this prior art arrangement, that portion of the brake pressure produced by the static control pressure would represent the lower limit capability of the control range of such ABS. This represents a considerable disadvantage of such ABS. In order to eliminate this particular disadvantage, such ABS includes a cutoff valve, designated as 19, in the connection located between the output of the master cylinder and the first control chamber of the servo cylinder. Such cutoff valve is set in the cutoff position at the beginning of an operation of such ABS. In this manner, the first control chamber is separated from the master cylinder and is connected to the auxiliary reservoir provided in such hydraulic brake system. The restriction of the lower limit capability of the control range on such ABS is thereby eliminated. However, as a result of this above-mentioned connection of the first control chamber to such auxiliary reservoir, the static control circuit is in an open condition. This open condition of the static control circuit represents what is considered in the art to be a rather severe disadvantage with respect to safety.
That portion of the brake pressure produced by the static control pressure in the prior art ABS, as described in such German publication No. DE 24 50 874 B2, disappears suddenly as a result of the activation of the cutoff valve at the beginning of an anti-lock operation. Further, such brake pressure produced by the static control pressure is restored suddenly upon return to normal operation. Additionally, this cutoff valve is one more source for a potential malfunction of such prior art ABS.